Carbonyl reductase activity accounts for a significant fraction of the metabolism of pharmacological agents extensively used in clinical practice such as the antipsychotic haloperidol and the anticancer anthracyclines doxorubicin and daunorubicin. In humans there are two carbonyl reductases, carbonyl reductase 1 (CBR1) and carbonyl reductase 3 (CBR3). It is possible that genetic variability in CBR1 and CBR3 may be key for the wide person-to-person variation in the metabolism of drugs that are CBR substrates. A systematic approach that combines the identification of common CBR1 and CBH3 genetic variants together with functional studies will be needed to critically delineate the role of CBR1 and CBR3 in variable CBR mediated drug biotransformation. Towards this goal we investigated the presence of single nucleotide polymorphisms (SNPs) in CBR1 and CBR3. Two SNPs in CBR3 encoding for non-synonymous changes in the amino acidic sequence of the protein were pinpointed to perform functional characterization studies. One SNP encodes for a valine244 to methionine244 change (CBR3 V244M), while the other SNP results in a cysteine4 to tyrosine4 substitution (CBR3 C4Y). Interestingly, the CBR3 allelic variants are common among different ethnic groups. Very promising kinetic data suggest that the polymorphic CBR3 valine244 and CBR3 methionine244 protein isoforms have distinctive catalytic properties towards menadione and doxorubicin. Thus, studies in specific Aim 1 will focus on the functional characterization of the polymorphic CBR3 protein isoforms. In specific Aim 2 the effects of polymorphic CBR3 in hepatic CBR activity will be investigated using 200 paired DNA-RNA liver tissue samples. The presence of genotype-phenotype correlations will be analyzed by measuring variables such as enzyme activities and protein levels in samples with known CBR3 genotypes. An additional contributing factor for inter-individual CBR variability may be dictated by the presence of genetic polymorphisms in the regulatory CBR1 and CBR3 proximal promoter regions. In specific Aim 3, DNA samples from phenotypic CBR outliers (pinpointed in Aim2) will be screened to identify new polymorphisms in the thus far unexplored CBR1 and CBR3 proximal promoter regions. Subsequent to the determination of allele frequencies, the functional consequences of the novel allelic variants will be examined by DNA expression analysis using gene reporter assays. Collectively, the proposed studies will provide essential information on the impact of CBR1 and CBR3 genetic variability on CBR mediated drug metabolism. The understanding of the molecular basis that govern the pharmacodynamics of CBR metabolized drugs will assist the design of more rational pharmacological therapies.